The present invention relates in general to a system for measuring the liquid level in a pressurized vessel, and more particularly a system for measuring the liquid level in a pressurized vessel that employs a differential pressure detector for sensing the head of a variable head column and a constant head column.
Liquid level detection and measurement systems perform more accurately during steady-state operating conditions. Steady-state errors result from density differences of liquids between calculated conditions and operating conditions of the variable column and the reference column. During variations in the steady-state operation, transient conditions are generated, which cause liquid level detection and measurement systems to provide inaccurate and misleading results. The transient conditions become of interest when a large pressurized vessel, such as a steam drum in fossil power plants or a steam boiler used in nuclear power plants, is employed for boiling a liquid. The pressurized vessel has the capability of rapid pressure increases and rapid pressure decreases while the liquid level in the pressurized vessel is measured.
Heated reference columns have been employed heretofore to compensate for the density changes in the pressurized vessel. the heated reference column maintained the temperature of the liquid in the reference column close to the temperature of the pressurized vessel to reduce density changes. Heated reference columns, however, are suject to flashing and boiling. Correcting for the density variations by heated reference columns increased the probability of flashing during variations in the steady-state conditions.
In the patent to Morooka, U.S. Pat. No. 4,389,888, issued on June 28, 1983, for Level Meter, there is disclosed a level meter for measuring the liquid level in a pressurized vessel. The liquid level employs a reference column having a condensing chamber, an active column and a differential pressure detector for indicating liquid level. Insulation has been added to the active column and the reference column, excepting the condensing chamber, to retard heating of the columns from an elevated ambient temperature in a drywell. A cooling water jacket is added to the uninsulated portion of the reference column below the condensing chamber to reduce flashing or rapid depressurization and to provide cooling for elevated drywell temperatures.
The patent to Kinderman, U.S. Pat. No. 3,132,516, issued on May 12, 1964, for Liquid Level Indicator, discloses a constant head chamber. A pressurized vessel communicates with the constant head chamber. The constant head chamber is provided with a variable head column and a heated reference column. A pressure differential indicator is connected to the variable head column and the heated reference column. Disposed above the liquid level of the constant head column is an auxiliary head chamber. The auxiliary head chamber communicates with the constant head chamber. In the even of a rapid pressure drop in the pressurized boiler, which causes flashing in the constant head chamber, water from the auxiliary head chamber flows into the constant head chamber to maintain a constant head during fluctuation. As water from the auxiliary head chamber flows into the constant head chamber, steam takes the place of water in the auxiliary head chamber.
The patent to Kinderman, U.S. Pat. No. 3,232,111, issued on Feb. 1, 1966, for Liquid Level Indicator, discloses a heated reference column which includes a constant head chamber. The liquid level indicator measures liquid level in a pressurized vessel with direct or indirect compensation for density changes of the liquid/vapor within the pressurized vessel. The constant head chamber is disposed below the lowest level of any indication to be measured on the gauge.
A variable column, a cold reference column including a condensing chamber, an instrument manifold, and a differential pressure detector are well-known in the remote measurement of liquid level in a pressurized vessel as disclosed in ASME Boiler and Pressure Vessel Code, Section L, paragraph PG-60 and in American Petroleum Institute Standard RP 550.